The third generation partnership project (3GPP) relates to standardization of the Long Term Evolution, LTE, concept. The architecture of the LTE system includes radio access nodes, eNBs, and interfaces to evolved packet core nodes, MME/S-GW. The interface between eNBs is referred to as X2, and the interface between eNB and MME is denoted S1.
From LTE Release-10, aggregation of carriers is supported with individual bandwidths up to 20 MHz (which is the maximal LTE Rel-8/9 carrier bandwidth). Hence, LTE Rel-10 allows for operations wider than 20 MHz. Such operations appear as a number of LTE carriers to an LTE user equipment operating in accordance with release 10. Carrier aggregation is an optional feature in release 10 and enhancements to this feature are added in 3GPP releases later than 10.
The release standard supports up to five aggregated carriers where each carrier is limited in the RF specifications to have one of six predefined bandwidths.
Carrier aggregation is called contiguous or non-contiguous intra-band if the carriers all belong to the same 3GPP operating band. Inter-band carrier aggregation is the case when there is at least one carrier in a different 3GPP operating band. The number of aggregated component carriers, CC, as well as the bandwidth of the individual CC, may be different for uplink and downlink. A symmetric configuration refers to the case where the number of CCs in downlink and uplink is the same, whereas an asymmetric configuration refers to the case when the numbers of CCs are different. However, an asymmetric configuration where the number of uplink CCs is higher than the number of downlink CCs is not allowed.
The number of CCs configured in the network may be different from the number of CCs seen by a user equipment. A user equipment may for example support more downlink CCs than uplink CCs, even though the network offers the same number of uplink and downlink CCs.
During initial access, an LTE release 10 user equipment behaves similar to an LTE release 8/9 user equipment. The serving cell, in which the UE ends up at initial access, is referred to as the UE's Primary Cell, PCell.
Upon successful connection to the network, a user equipment may—depending on its own capabilities and the network—be configured with additional CCs in the UL and DL. These additional serving cells, which are configured for the UE, are referred to as Secondary Cells, SCells.
The Primary cell, Pcell, basically corresponds to the release 8/9 “serving cell”. The UE monitors system information only on the PCell cell and also takes security and Non-access stratum, NAS, mobility information from this cell. System information needed for the SCell is provided via dedicated signalling. The SCell does not have any Physical Uplink Control Channel, PUCCH, of its own. The UE may transmit Channel State Information, CSI, for SCells on the PUCCH of the PCell. Cross-carrier scheduling (i.e. being scheduled from a Physical Downlink Control Channel, PDCCH, of another serving cell used by the same UE) is not supported for the PCell, while Semi-Persistent Scheduling, SPS and Transmission Time Interval, TTI, bundling is only supported on the PCell. Also Radio Link Monitoring and Radio Link Failure only applies to the PCell.
The PCell can be changed using handover, HO. During HO all SCells configured for the UE doing the HO will be deactivated. The target eNB (may be the same as source eNB) will then decide whether to use the same SCells, configure and activate a different set, or to simply reconfigure them.
The PCell is always activated, when the UE is Radio Resource Control, RRC, connected, while SCells can be activated and deactivated on a need basis. Activation/Deactivation of SCells is performed using a Medium Access Control Element, MAC CE, while configuration/deconfiguration is performed using RRC signalling.
To make a downlink transmission on a configured SCell or to be able to provide a UE in the SCell with an uplink grant the eNB first activates the SCell by sending an activation/deactivation command (as a MAC CE) to the UE.
When a user equipment is activated on multiple SCells this would imply it has to monitor all DL CCs for PDCCH and Physical Downlink Shared Channel, PDSCH. This implies a wider receiver bandwidth, higher sampling rates, etc. resulting in high power consumption.
A UE capable of carrier aggregation can have only one PCell and up to four SCells. The Cell-Radio Network Temporary Identifier, C-RNTI, is UE specific and the same C-RNTI is used both in the PCell and in the SCell(s).
In LTE FDD systems, most frequency bands are defined with one uplink band and one downlink band, to facilitate two-way communication. The X2 interface is available between eNBs in LTE. When an X2 link is established, information on the served cells is exchanged between the two eNBs. Thus, the frequency band used for the respective cell is communicated. For cells using FDD, a DL and an UL frequency is signalled, for all frequency bands defined as DL and UL. For frequency bands defined as DL-only in the 3GPP standards, a dummy value is given for the UL frequency.
In the special case of carrier aggregation, a UE is connected to a cell using a set of frequencies but can also simultaneously use a second cell using another set of frequencies. The second cell may by the UE either be used for both UL and DL transmission or only be used for DL transmission. Typically, the cell used as a secondary cell, has both an UL and a DL carrier from an eNB point of view even if the UE is configured to utilize e.g. only the DL carrier of this cell. Then the eNB will still have an UL frequency configured for this cell and listen to it, even if not all UEs are configured to have any UL transmission on this frequency even when the UE is using the DL frequency of the cell for data transmission.
There are however special cases where a cell from an eNB point of view can be deployed with only a DL (this is not to be confused with the case when a UE is configured to only use the DL of a cell). This means that the eNB is typically not listening on the UL frequency associated with the DL frequency of this cell or ignoring it, alternatively the DL frequency has no associated UL frequency.
In the 3GPP standards there is currently only one band (band 29) defined as DL-only, meaning that it has no uplink EUTRAN Absolute Radio Frequency Channel Number, UL-EARFCN, defined, only DL-EARFCN. Other bands are defined in the 3GPP standards with both UL-EARFCN and DL-EARFCN. For the operator, these could be desirable to use for a DL-only cell. Cells deployed as DL-only cells can be utilized for the purpose of secondary cells providing additional resources for DL transmission.
The IE Served Cell Information shown in the table below contains cell configuration information of a cell that a neighbour eNB may need for its operation. The table below is copied from version V.11.5.0 of 36.423.
As can be noted, the IE states that information provided in UL EUTRAN Absolute Radio Frequency Channel Number, EARFCN, field shall be ignored for operating bands which are defined as DL-only bands by 3GPP in 36.104. This IE can thus be used to signal that band 29 is DL-only, but for other bands which have a defined UL, there is no current way to provide this information to the neighbour eNB.
TABLE 1The Served Cell Information IE as specified in 3GPP TS 36.423IE typeandAssignedIE/Group NamePresenceRangereferenceSemantics descriptionCriticalityCriticalityPCIMINTEGERPhysical Cell ID——(0..503, . . . )Cell IDMECGI——9.2.14TACMOCTETTracking Area Code——STRING(2)Broadcast1 . . . <maxnoofBPLMNs >Broadcast PLMNs——PLMNs>PLMN IdentityM9.2.4——CHOICEM——EUTRA-Mode-Info>FDD>>FDD Info1——>>>UL EARFCNMEARFCNCorresponds to NUL in TS——9.2.2636.104 [16] for E-UTRAoperating bands forwhich it is defined;ignored for E-UTRAoperating bands forwhich NUL is not defined>>>DL EARFCNMEARFCNCorresponds to NDL in TS——9.2.2636.104 [16]>>>ULMTransmissionSame as DL——TransmissionBandwidthTransmission BandwidthBandwidth9.2.27in this release; ignoredin case UL EARFCN valueis ignored>>>DLMTransmission——TransmissionBandwidthBandwidth9.2.27>>>UL EARFCNOEARFCNIf this IE is present, theYESrejectExtensionExtensionvalue signalled in the UL9.2.65EARFCN IE is ignored.>>>DL EARFCNOEARFCNIf this IE is present, theYESrejectExtensionExtensionvalue signalled in the DL9.2.65EARFCN IE is ignored.>TDD——>>TDD Info1——>>>EARFCNM9.2.26Corresponds to NDL/NUL——in TS 36.104 [16]>>>TransmissionMTransmission——BandwidthBandwidth9.2.27>>>SubframeMENUMERATEDUplink-downlink——Assignment(sa0,subframe configurationsa1, sa2,information defined insa3, sa4,TS 36.211 [10]sa5,sa6, . . . )>>>Special1Special subframe——Subframe Infoconfigurationinformation defined inTS 36.211 [10]>>>>SpecialMENUMERATED——Subframe(ssp0,Patternsssp1, ssp2,ssp3, ssp4,ssp5, ssp6,ssp7, ssp8, . . . )>>EARFCNO9.2.65If this IE is present, theYESrejectExtensionvalue signalled in theEARFCN IE is ignored.>>>>CyclicMENUMERATED——Prefix DL(Normal,Extended, . . . )>>>>CyclicMENUMERATED——Prefix UL(Normal,Extended, . . . )>>>AdditionalOSpecial subframeGLOBALignoreSpecialconfigurationSubframe Infoinformation defined inTS 36.211 [10]. Only fornewly definedconfiguration of specialsubframe from Release11.>>>>AdditionalMENUMERATED——Special(ssp0,Subframessp1, ssp2,Patternsssp3, ssp4,ssp5, ssp6,ssp7, ssp8,ssp9, . . . )>>>>CyclicMENUMERATED——Prefix DL(Normal,Extended, . . . )>>>>CyclicMENUMERATED——Prefix UL(Normal,Extended, . . . )Number ofO9.2.43YESignoreAntenna PortsPRACHOPRACHYESignoreConfigurationConfiguration9.2.50MBSFN0 . . . <maxnoofMBSFN>MBSFN subframeGLOBALignoreSubframe Infodefined in TS 36.331 [9]>RadioframeMENUMERATED——Allocation(n1,Periodn2, n4, n8,n16, n32, . . . )>RadioframeMINTEGER——Allocation Offset(0 . . . 7, . . . )>SubframeM9.2.51——AllocationCSG IDO9.2.53YESignoreMBMS Service0 . . . <maxnoofMBMSServiceAreaIdentities >Supported MBMSGLOBALignoreArea IdentityService Area Identities inListthe cell>MBMS ServiceOCTETMBMS Service AreaArea IdentitySTRING(2)Identities as defined inTS 23.003 [29]MultibandInfoListO9.2.60YESignoreRange boundExplanationmaxnoofBPLMNsMaximum no. of Broadcast PLMN Ids. Value is 6.maxnoofMBSFNMaximum no. of MBSFN frame allocation with different offset. Value is 8.maxnoofMBMSServiceAreaIdentitiesMaximum no. of MBMS Service Area Identities. Value is 256.
The EUTRAN Absolute Radio Frequency Channel Number, EARFCN, Extension defines the carrier frequency used in a cell for a given direction (UL or DL) in FDD or for both UL and DL directions in TDD. It includes Extended EARFCN including allowed range.
TABLE 2EARFCN including allowed range describedin section 9.2.26 of 36.423 (V.11.5.0)IE/GroupPres-IE Type andSemanticsNameenceRangeReferenceDescriptionEARFCNMINTEGERThe relation between(0 . . .EARFCN and carriermaxEARFCN)frequency (in MHz)are defined inTS 36.104 [16].Range boundExplanationmaxEARFCNMaximum value of EARFCNs. Value is 65535.
TABLE 3Extended EARFCN including allowed range as describedin section 9.2.65 of 36.423 (V.11.5.0)IE/GroupPres-IE Type andSemanticsNameenceRangeReferenceDescriptionEARFCNMINTEGERThe relation betweenExtension(maxEARFCN +EARFCN and carrier1 . . .frequency (in MHz)newmaxEARFCN,are defined in. . .)TS 36.104 [16].Range boundExplanationmaxEARFCNMaximum value of EARFCNs. Value is 65535.newmaxEARFCNNew maximum value of EARFCNs. Value is262143.
To sum up, in Long Term Evolution, LTE, Frequency Division Duplex, FDD, systems, most frequency bands are defined with one uplink band and one downlink band, to facilitate two-way communication. The X2 interface is available between evolved Node Bs, eNBs, in LTE. When an X2 link is established, information on the served cells is exchanged between the eNBs. Then, the frequency band used for the respective cell is communicated between the eNBs. For cells using FDD, a downlink, DL, and an uplink, UL, frequency is signalled for all frequency bands defined as DL and UL. For frequency bands defined as DL-only, a dummy value is given for the UL frequency.